There are many situations in which high temperature filters are utilized to clean gases by removing particles from the gases. One of the most significant applications of high temperature filters (such as ceramic filters) is at the filtering of small particles from high temperature gases produced during the pressurized combustion or gasification of carbonaceous fuels, such as coal. Some of the high temperature filters utilized for this environment are shown in U.S. Pat. Nos. 4,343,631, 4,417,908, 4,525,184, 4,904,287, the disclosures of which are hereby incorporated by reference herein. Typically the filtering elements comprise ceramic candle filters, ceramic honey combs, ceramic tubes, or alternating layers of gas permeable ceramic membranes corrugated sheets.
Standard procedure for cleaning ceramic filters in these environments is to provide periodic backflushing of the ceramic filters utilizing a high pressure gas stream. Such a system, if employed effectively, such as in the manner that was created in some of the above identified patents, successfully keeps the filters per se clean enough to effectively perform the filtering function over relatively long time periods. However, it has been found that especially for filtering systems used with pressurized fluidized bed combustion or gasification of solid carbonaceous fuel (such as coal), where gases having a temperature of over about 400.degree. C. and at a pressure of about 5 bar or above are produced, and they are heavily laden with particles, ceramic filters fail more quickly than predicted or desired. According to the present invention it has been found that the result of this failure is a high buildup of particles on surfaces supporting and/or surrounding the filters, a phenomenon that may be denoted "ash bridging". This buildup of particles can result in high lateral forces that are applied to the filter elements, or can result in gradual clogging of some of the filter surfaces.
The problem of ash bridging is not solved by conventional backflushing used to clean the filters, and as a matter of fact the backflushing may under some circumstances even intensify the ash bridging problem by directing very high velocity particles at supporting or surrounding surfaces. Also the thermal gradients that can often occur during backflushing may, in combination with the ash bridging, accelerate filter failure, particularly for ceramic candle filters.
According to the present invention, not only has the problem of ash bridging been identified in association with ceramic filters in high temperature environments, particularly for ceramic filters where the gas to be filtered is at high temperature and pressure and heavily particle laden (e.g. as from the pressurized fluidized bed combustion or gasification of coal), typically at temperatures above about 400.degree. C. and pressures above about 5 bar, but the invention also solves the problem.
The invention comprises both a method and apparatus for solving the problem of particle buildup and ash bridging associated with filtering systems for high pressure and temperature gases. The invention solves the problem by automatically cleaning the supporting and surrounding surfaces. While this cleaning may be accomplished in different manners, a preferred mechanism for effecting cleaning is to direct high pressure streams of compressed gas, steam, or a like fluid at the surfaces where buildup is likely to occur. The fluid streams are directed at spaced points and times, e.g. periodically, intermittently, or in response to sensing of the flow volume of gases, or other conditions that might be indicative of buildup of particles.
According to one aspect of the present invention, a method of operating a pressurized gas filtering system comprising a plurality of filtering elements having a first side for contact with particle laden gas, and a second side through which filtered gas passes, and mounted by and/or adjacent to supporting and supporting surfaces is provided. The method comprises the steps of: (a) substantially continuously introducing superatmospheric pressure particle laden gas into contact with the first side of the filtering elements so that the gas passes through the filtering elements, and the filtering elements remove particles therefrom, (b) while practicing step (a) withdrawing filtered gas from the second side of the filtering elements, and (c) at spaced points in time automatically cleaning the supporting and surrounding surfaces of filtering elements to prevent buildup of particles on the supporting and/or surrounding surfaces that would adversely act upon the filtering elements. There preferably is also the further step (d) of, at spaced points in time, backflushing the filtering elements by directing a pulse of cleaning gas through the filtering elements from the second side to the first side at a pressure greater than the pressure of the gas introduced in step (a).
Step (a) is typically practice by introducing gas at a pressure of about 5 bar or more and at a temperature of about 400.degree. C. or more, such as gas from the pressurized fluidized bed combustion or gasification of solid carbonaceous fuel such as coal. Step (c) is preferably practiced by directing pulses of fluid at the supporting and/or surrounding surfaces, the fluid at a high enough pressure to effect removal of particles from the supporting and/or surrounding surfaces, the fluid typically comprising compressed gas (e.g. air or an inert gas such as nitrogen or argon) or steam. Step (c) is advantageously practiced periodically, or intermittently. Steps (c) and (d) may be practiced substantially simultaneously, or at different intervals.
The filtering elements may comprise ceramic candle filters mounted by mounting collars having a first dimensions of elongation and connected to a first substantially planar wall structure generally perpendicular to the first dimension, and wherein some of the mounting collars may be adjacent a second wall structure generally parallel to the first dimension, corners being provided between the mounting collars and first wall structure, and between the first and second wall structures. In this case the practice of step (c) typically includes directing the fluid directly into the corners.
According to another aspect of the present invention a superatmospheric pressurized filter and assembly is provided. The assembly comprises the following elements: A pressure vessel having an inlet for superatmospheric pressure particle laden gas, and an outlet for clean, filtered gas. A plurality of filter elements each having a first side for contacting particle laden gas, and a second side through which filtered gas flows. Means for mounting the filter elements within the vessel so that particle laden gas flows through the first sides of the elements, particles collecting thereon, the means including a supporting structure for the filter elements, and surrounding structures adjacent to some of the filter elements. And means for automatically directing superatmospheric pressure fluid at the supporting and/or surrounding structures to prevent buildup of particles on the surfaces which would adversely affect operation of the filter elements.
The pressurized fluid directing means typically comprises a plurality of fluid conduits extending adjacent the supporting and/or surrounding structures, and having nozzles formed therein. If the filters are ceramic candle filters--as described above--the fluid directing means may include means for directing the pressurized fluid directly into the corners. The fluid directing means typically further includes an automatically operated valve disposed in a pipe connecting the fluid conduits to a source of gas under superatmospheric pressure, and means for periodically operating the valve.
Typically the inlet to the pressure vessel is connected to a pressurized circulating fluidized bed reactor for combusting or gasifying solid carbonaceous fuel. A cyclone separator, or like "large" particle separator, is normally provided between the reactor and the filter means.
The supporting means typically comprises a generally vertical wall structure from which a filter element extends generally horizontally--when the filter elements are honeycomb ceramic filters, or the like--and the fluid conduits are mounted on the generally vertical wall structure. Backflushing means are typically also provided for periodically directing high pressure clean fluid into contact with the second side of the filter elements to pass through the filter elements and dislodge particles which collect on the actual filtering surfaces (that is on the filter elements first side).
According to yet another aspect of the present invention a method of operating a fluidized bed reactor and a plurality of filtering elements mounted by supporting structures in operative association with gases from the reactor is provided. The method comprises the steps of substantially continuously and sequentially: (a) chemically reacting carbonaceous fuel in the reactor to produce a particle-laden gas at a temperature greater than about 5 bar and a temperature of greater than about 400 degrees C., (b) removing large particles from the particle-laden gas, (c) passing the gas into contact with the filtering elements so that particles are separated from the gas, and (d) periodically directing clean fluid at a pressure greater than the pressure of the gas in step (a) directly at the supporting structures for the filter elements to remove particles therefrom, and prevent buildup of particles thereon.
It is the primary object of the present invention to provide more effective filtering of particle laden gases, particularly in superatmospheric pressure and high temperature environments, or other situations in which ash bridging can occur. This and other objects of the invention will become clear from an inspection from the detailed description of the invention and from the appended claims.